I think you guys have missed the point! My observation was that the hump in the middle of the housing to clear the divider is effectively blocking 1/3 of the element surface area!!

The design does not draw air from the top of the engine but along side it, above the exhaust manifold (which is probably worse)! Cold air is definitely worth the effort and on a 6.9 there are several options. Something interesting which I forgot about (until I saw one today) is the RH guard intake duct on a W124 diesel....

I've heard the same about K&N filters and the quality isn't as good as a MB or OEM one.

The best conversion I have seen was to go the other way and draw the air from the ducting/grill below the windshield. The air cleaner was ducted into the firewall, allowing it to draw down cold air from the grill.

4 years 7 months since I felt this was an issue and it's finally be proven true on the dyno by Carobu!!! And rather than make a lot of changes to the standard assemblies, I intend to use one from a W140. But even that may need some (more minor) custom mods....

Oh and apparently the air valve baffle (stopper bridge) also restricts air flow.

Been chasing knowledge about Mercedes C Class W204 Air Filters which are as good as they come. No advantage fitting K & N washable Filter
Biggest improvement has been resetting The ECU and the TCU back to how they left the Factory. I now have a part throttle downshift, crisper upshifts and responsive Throttle. I guess Mercedes know that they are doing !!!!